TGFp superfamily growth factors, including activins, myostatin, and bone morphogenetic proteins (BMPs), have numerous roles in development and adults and are often dysregulated in diseases ranging from cancer to infertility. Activin and myostatin are regulated by soluble antagonists, including follistatin (FST) and follistatin like-3 (FSTL3). FST is critical for normal development since FST KO mice die within 24 hours of birth. FSTL3 has many biochemical and functional similarities to FST and a partially overlapping expression pattern, suggesting some degree of compensatory actions. Our preliminary data demonstrate that the, three FST protein isoforms and FSTL3 have different biochemical characteristics that contribute to distinct bioactivities and mechanisms in vitro. To explore the function of the FST isoforms in vivo, we created a knock-in mouse model in which the circulating FST315 isoform was deleted (FST288-only). This mutant mouse developed of a suite of reproductive and metabolic phenotypes, including enlarged pancreatic islets with p-cell hyperplasia, enhanced glucose tolerance and insulin sensitivity, reduced visceral fat, and upregulated hepatic neogenesis. FST288-only mice are also sub fertile and cease ovarian activity prematurely. Our existing FSTL3 KO mouse has many of these same metabolic defects, but distinct reproductive phenotypes. The Broad Goal of this proposal is to determine the mechanisms by which FST315 deletion produces specific reproductive and metabolic alterations. Our overall hypothesis is that FST315 has critical roles in regulating activin, myostatin, and/or BMP actions that are required for normal reproduction and metabolism but are not fulfilled by the FST288 isoform. Specific Aim 1 will elucidate mechanisms where deletion of FST 315 and FST303 leads to activin/BMP mediated subfertility while Aim 2 will examine the role of activin and myostatin in regulating p-cell proliferation, enhanced peripheral insulin sensitivity, and hepatic insulin response. In Specific Aim 3 we will combine these mouse models to determine the degree of functional compensation between FST315/303 and FSTL3 in regulating these important actions of activin and myostatin. Our proposed studies could lead to novel therapies for type 1 and type 2 diabetes and insulin resistance, as well as identify new mechanisms for POF. The number of people diagnosed with obesity and insulin resistance is increasing at an alarming rate, particularly in children, creating a need for new treatments. This research opens a new pathway that could be used to identify new drugs to treat diseases like type I and 2 diabetes and infertility in humans.